The present invention relates generally to delay lines for delaying light signals, and more particularly to a compact delay line for matching phases between two light signals in an interferometer.
Referring to FIGS. 1 and 2, an interferometer 15 has many applications including determining the distance between two light sources (not shown) which radiate two separate light signals 16, 17. To do so, the interferometer 15 produces an interference fringe pattern 18 by equalizing the path length between the first 16 and second 17 light signals by phase delaying the first light signal 16 by an amount equal to the path length difference L between the first 16 and second 17 light signals. An interferometer 15 typically uses two separate collecting telescopes 19, 20 to collect the two light signals 16, 17 respectively. The collecting telescopes 19, 20 route the two light signals 16, 17 to a beam combiner and focal plane 21 where the interference fringe pattern 18 is formed. To equalize the pathlengths between the light signals 16, 17, a three stage delay line 22 is placed in one or both legs 23, 24 of the interferometer 15 and is configured to balance the pathlengths between the two light signals 16, 17. Fixed mirrors 25, 26 are used to provide ingress and egress for the first light signal 16 to enter the three stage delay line 22. The first stage 27 of the three stage delay line 22 is used for correcting path length errors in the meter to millimeter range and typically uses moveable mirrors 28, 30 to adjust the length of the delay path 32. To reduce or increase the delay path 32, the adjustable mirrors 28, 30 are moved closer to 33 or further from 34 the fixed mirrors 25, 26 respectively.
The second stage (not shown) of the three stage delay line 22 is used to correct path length errors in the millimeter to micron range and typically uses a voice coil to provide micron accuracy. The third stage (not shown) of the three stage delay line 22 is used for correcting path length errors in the micron and nanometer range and typically uses a piezoelectric material to provide nanometer accuracy.
The path length adjustment capability of the first stage 27 is limited by the physical travel capability of the adjustable mirrors 28, 30, since the absolute amount of phase error that can be corrected in the interferometer 15 is limited by the path length adjustment capability, or throw, of the first stage 27 of the three stage delay line 22. This can cause a problem for interferometers requiring large path delay capabilities such as interferometers with large angular fields of view and space-based free-flying interferometers.
For a space-based free-flying interferometer, the collectors 19, 20 of the interferometer 15 are located on separate spacecraft with the combiner 21 and the three stage delay line 22 being located on a third spacecraft. The phase error correction capability of the first stage 27 of the three stage delay line 22 mandates the accuracy within which the interferometer collectors 19, 20 must be controlled. This in turn constrains the formation flying accuracy in which the spacecraft must be controlled. For example, a free-flying interferometer 15 having a ten centimeter long throw 32 would constrain the spacecraft to flying in formation with errors of less than ten centimeters which is a potentially stressing requirement on the system. Increasing the path length 32 of the three stage delay line 22 can allow larger angular movement and ease the stringent formation accuracy requirement. However, this can be difficult in the typical three stage delay line 22 since increasing the path length of the first stage 27 of the typical three stage delay line 22 requires increasing the spacing between the mirrors 25, 26, 28, 30, requiring additional area on the spacecraft consuming spacecraft volume which may not be available and can be expensive.
What is needed therefore, is a delay line with the capability to provide increased delay to a light signal 16 and do so in a compact space.